1. Field of the Invention
This invention relates in general to voltage level detecting circuits, and more particularly, for monitoring the D.C. voltage level in a battery and A.C. power to a charger for maintaining a charge on the battery.
2. General Background
A disadvantage in systems presently in use is that many systems utilize power from the battery being monitored to power the circuitry used as a reference voltage for monitoring the voltage in the battery. This is a disadvantage in that it drains the main battery of useful life and should the main battery fail suddenly, there is no power in the system for activating an alarm. Also, if a system utilizes only A.C. power as a means for providing a reference voltage, the failure of A.C. power leaves the system without backup to provide a reference voltage to the circuitry. Voltage monitoring systems of which applicant is aware include the following.
U.S. Pat. No. 3,969,697, issued to H. Iwasa et al, entitled "Voltage Drop Warning Apparatus with Negative-Resistance Device," teaches battery monitoring circuitry that senses when a battery is failing and activates an alarm. A series connection comprising a transitor and a light emitting diode is connected across a D.C. power source. One end of a negative resistance device is connected to the base of the transistor and the other end is connected to the end of the light emitting diode, which is connected to the D.C. power source. The circuit memorizes an occurrence of voltage drop which enables bi-stable switching in the negative resistance device, thereby energizing the LED. The circuit can be used to indicate voltage lowering or interruption of an A.C. power supply or weakening of a battery.
U.S. Pat. No. 4,017,724, issued to E. P. Finger, entitled "Apparatus For Measuring Battery Depletion By Monitoring Reductions In Voltage", teaches a battery monitoring circuit that senses when a battery is failing and activates an alarm. The apparatus comprises means for sensing reductions in the output terminal voltage of the battery due to varying load conditions and produces a signal in response thereto during the time when the voltage terminal is below a threshold value. The apparatus also comprises an integrater such as an electronic counter or a stepping motor for integrating the signal and means for displaying the integral accumulated.
U.S. Pat. No. 4,119,904, issued to S. A. Haglund, entitled "Low Battery Voltage Detector," discloses a circuit for detecting a low voltage condition and a battery comprising a MOSFET, an inverter, means for connecting the MOSFET to one terminal of the battery, means for connecting the source of the MOSFET to the other terminal of the battery, means for connecting the drain of the MOSFET to one terminal of the battery into the input of the inverter and feedback means for connecting the output of the inverter to the gate of the MOSFET. A predetermined low voltage condition in the battery causing a decrease in voltage at the gate of the MOSFET when this voltage drops below the threshold value of the MOSFET turns the MOSFET off, thus producing an increased voltage at the input of the inverter and a decreased voltage at the output of the inverter which is fedback to further decrease the voltage and cause rapid shutoff of the MOSFET, producing an indication of the low voltage condition in the battery.
U.S. Pat. No. 3,778,800, issued to L. L. Blackwell, et al, entitled "Self-Monitoring Battery Operated Circuit," discloses a time pulse generator connected to a battery being monitored. The output of the time pulse generator drives a signalling device to indicate a low level battery condition and/or an alarm condition as detected by an external sensor.
U.S. Pat. No. 4,224,539, issued to F. H. Musa, et al, entitled "FET Voltage Level Detecting Circuit," discloses a field effect transitor (FET) voltage level detector having a constant voltage reference. The voltage level detector is made as a monolithic integrated circuit and has a constant voltage reference generator having at least three field effect transitors connected in series, wherein the first transitor is a depletion device and the second and third transitors are enhancement devices having their gate electrodes connected to their drain electrodes. An output of the constant voltage reference generator is provided by a junction formed by the transistors. There is also provided a power supply voltage sensor having a plurality of resistances coupled between the terminals of the power supply. The constant voltage reference generator provides a constant voltage which does not vary with variations of the power supply voltage while the power supply voltage sensor varies in relation to the variations of the power supply. Outputs of the constant voltage reference generator and the power supply voltage sensor can be coupled to a comparator so that the comparator will provide an output whenever the voltage sensed by the power supply voltage sensor reaches a predetermined level.
U.S. Pat. Nos. 3,786,342; 4,025,700; 4,138,664; 4,030,086; 4,216,648; and 3,778,800 disclose circuits for monitoring a battery operated circuit, indicating battery discharge or warning devices or multi-voltage step batteries and are representative of what is in the art.